Hmm...
I vaguely remember the ads for "Reign Storm", and at that time, I was still a newcomer to the DP universe (though I knew the majoity of the charaters), but I remember still being SOOOOOO excited for a couple of reasons:
1.) I seem to have this absolute LOVE for stories that involve pretty much every character needing to come together to defeat a bigger threat, even if a few of the characters hate each other's guts. The ads showed Danny talking to Vlad, saying "You're helping ME?!", and shaking hands with Valerie while saying "You don't have to trust me, just fight with me!", and you had NO IDEA how pumped up that made me.
2.) By this time, I had admitted to myself that I might've possibly had the tiniest, TINEST crush on Danny (yeah, Denial-Land was pretty nice that time of year

), so seeing a new episode of him was already pretty exciting even without the plot device mentioned above.
3.) C'mon, think about it! The ads showed Amity Park practically being engulfed by the Ghost King, THE FRIGGIN' KING OF ALL THINGS GHOSTLY! If that wasn't exciting, I don't know WHAT could be.
So I was REALLY looking foward to it, and actually watching RT helped turn me into the fan I am today.
Not only was the storyline epic, and the subplots exciting (Vlad and Valerie, Vlad and the Fright Knight, Danny and Valerie's budding relationship), it still didn't fail at being funny as heck from time to time. ("You okay? ...Sorry, standard question." "Oh, sugar cookie!" "But more than anything, know this: THIS WAS NOT MY FAULT! I REPEAT, THIS WAS
NOT MY FAULT!!") By the time this was over, I was pretty speechless. This was heck of a ride, and it marked the realization that this was a series I wanted to seriously follow. To me, this whole episode was a peak of excitement that no other DP episode could made me feel ever again. Not even "The Ultimate Enemy" could compete with it. (Sorry, TUE fans!) But I enjoyed the rest of the series regardless.
Not to mention, it was the episode that made me finally come to grips with my feelings for Danny, but that's another story...
Um...don't remember if any other episode previews left an impact...I'll get back to you if there was another. ^^